This application claims the priority of Korean Patent Application No. 10-2004-0101615, filed on Dec. 6, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates, in general, to a layout structure of a semiconductor device and, more particularly, to a layout structure of a signal driver.
2. Description of the Related Art
Generally, a signal driver, such as an output driver, generates two output signals, which swing in opposite phases. In this case, the two output signals are required to have a wide output signal interval (that is, an interval in which a voltage difference is maintained above a certain level) so as to ensure the range that allows a signal value to be recognized.
FIG. 1 is a circuit diagram of a conventional signal driver 100, and FIG. 2 is a view showing the layout structure of the signal driver 100 of FIG. 1. The signal driver 100 of FIGS. 1 and 2 includes a first signal response unit 110, a second signal response unit 120 and a current source unit 130. Further, the current source unit 130 includes first to fourth bias units 131 to 134. MOS transistors implementing the four bias units 131 to 134, respectively, are constructed to have different W/L values (W: transistor channel width, L: transistor channel length). Therefore, source currents flowing through the first and second signal response units 110 and 120 can be variously restricted by respective bias control signals BIAS1 to BIAS4. The intrinsic resistance that forms between any two neighboring bias units, 131 to 134, is symbolized by reference items RS1, RS2 and RS3 in FIG. 2.
The bias units 131 to 134 of the conventional signal driver 100 are horizontally asymmetrically arranged, as shown in FIG. 2. Therefore, the skew in distance between the first and second signal response units 110 and 120 and the bias units 131 to 134 are generated, so that differences in actual resistance values exist.
In the layout structure of the conventional signal driver 100, current mismatch occurs between the first and second signal response units 110 and 120 with respect to the bias units 131 to 134, that is, the current source unit 130. The layout structure of the conventional signal driver 100 is of concern in that operating characteristics, such as output signal intervals, are deteriorated.